Plasma display panel apparatus having multilevel stable states for variable intensity

ABSTRACT

A plasma display panel apparatus having multilevel stable states for providing a display of variable intensity, the apparatus including means for initially setting in an amount of wall charge in each respective cell according to the intensity of the information to be displayed, and means for providing a sustaining signal for displaying the cells and maintaining the respective wall charges therein, the sustaining signal waveform having alternate stable and unstable regions. The means for setting in of the initial wall charge in respective cells is provided such that the cells having information which is to be displayed with a higher intensity are supplied with a higher initial wall charge than the cells having information which is to be displayed at a relatively lower intensity. The level of intensity depends on the slope of the exciting or sustaining signal at the time of discharge of the cell. The apparatus providing a rippled sustaining signal of alternate stable and unstable regions insures that the initial wall charge information set into the display panel will remain at the initial level, thereby providing a constantly recurring permanent display.

United States Patent [72] Inventors Donald L. Bitzer Urbana; Hiram GeneSlottow, Urbana; William Petty, Champaign, all of, I11.

[211 App]. No. 765,939

[22] Filed Oct. 8, 1968 [45} Patented Aug. 24, 1971 [73] AssigneeUniversity of Illinois Foundation Urbana, Ill.

[54] PLASMA DISPLAY PANEL APPARATUS HAVING MULTILEVEL STABLE STATES FORVARIABLE INTENSITY 8 Claims, 11 Drawing Figs.

[52] US. Cl l78/7.3 D,

[51] Int. Cl H04n 5/66 [50] Field ofSearch 315/167,

168,169, 169 TV; 178/67 A, 6.7, 7.3 D, 7.5 D, 6 A

[56] References Cited UNITED STATES PATENTS 2,595,617 5/1952 Toolon178/6 A 2,994,011 7/1961 Belknap et a1. 315/169 7 SYNC.

34-susrlunsn WALL cgAges l l l INFO. g 1 TO BE DlSPLAYED 1 l l LL CHARGE3,048,824 8/1967 Thompson 315/169 3,356,898 12/1967 Dano 315/1693,379,831 4/1968 Hashimoto 178/73 D Primary ExaminerRichard MurrayAssistant Examiner-Richard P. Lange Attorney-Merriam, Marshall, Shapiro& Klose ABSTRACT: A plasma display panel apparatus having multilevelstable states for providing a display of variable intensity, theapparatus including means for initially setting in an amount of wallcharge in each respective cell according to the intensity of theinformation to be displayed, and means for providing a sustaining signalfor displaying the cells and maintaining the respective wall chargestherein, the sustaining signal waveform having alternate stable andunstable regions. The means for setting in of the initial wall charge inrespective cells is provided such that the cells having informationwhich is to be displayed with a higher intensity are supplied with ahigher initial wall charge than the cells having information which is tobe displayedat a relatively lower intensity. The level of intensitydepends on the slope of the exciting or sustaining signal at the time ofdischarge of the cell. The apparatus providing a rippled sustainingsignal of alternate stable and unstable regions insures that the initialwall charge information set into the display panel will remain at theinitial level, thereby providing a constantly recurring permanentdisplay.

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mvemoas L DONALD L. an'zen H. GENE SLOTTOW WILLIAM D. PETTY ATTYS.

PATENIEU mm I9?! sum 2 or 3 3 4 SUSTAINING SIGNAL voLTAsEJ =0 WALLVOLTAGE FOR UNPERTURBED STATE WALL VOLTAGE FOR PERTURBED STATE FIG.5

R an n M T LA ml T B O 0 V T E NL L E L V0 A3 L WA mA T 6 N S 3 O o D M2 R M2 M O a s U T E M 0S 6 fi. M E v 0 N6 E G A M mm H M m U W5 V P V fvnw. v v 6 n Y p s 0b 2 I V a V 4 a. sane sLo'r-row WILLIAM D. PETTY w;wwzi w ATTYS.

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INVENTORS DONALD L. BITZER H. GENE SLOTTOW WILLIAM D. PETTY PLASMADISPLAY PANEL APPARATUS HAVING MULTILEVEL STABLE STATES FOR VARIABLEINTENSITY This invention relates to gaseous discharge devices, and inparticular to a gaseous discharge display panel having multilevel stablestates for providing variable intensity.

The subject matter of the present invention is related to apparatusdisclosed in a copending application of Donald L.

Bitzer, H. Gene Slottow and R. H. Willson, U.S. Ser. No. 1

plasma display panel." Reference may also be had to the followingpublications disclosing the type of plasma panel related to the presentinvention, such publications being incorporated herein in theirentirety;

l. Bitzer, D. L. and Slottow, H. G. The Plasma Display PanelA DigitallyAddressable Display with Inherent Memory, Proceedings of the Full JointComputer Conference, San Francisco, California, Nov. 1966.

v 2. Arora, B. M., Bitzer, D. L., Slottow, H. G., and Willson, R. H.,The Plasma Display Panel-A New Device for Information Display andStorage, Proceedings of the Eighth National Symposium of the Society forInformation Display, May 1967 3. Bitzer, D. L. and Slottow, H. G. ThePlasma Display Panel-A New Device for Direct View of Graphics,Conference on Emerging Concepts in Computer Graphics, University ofIllinois Nov. 1967, to be published by Benjamin Publishing Company, NewYork.

4. Bitzer, D. L. and Slottow, H. 6., Principles and Applications of thePlasma Display Panel, Proceedings of the OAR Research ApplicationsConference, Office of Aerospace Research, Arlington, Va., Mar. 1968.(Also published in the Proceedings of the 1968 MicroelectronicsSymposium, I.E.E.E., June 1968.

It is to be understood that the terms plasma panel or plasma displaypanel" is defined by and is characterized by the gaseous discharge panelas described in the previously mentioned copending application and abovelisted publications.

This application is concerned with apparatus and a method for providinga new variable intensity technique for the plasma display panel. Thistechnique is based on a stability theory for the plasma display panelwhich is discussed in the following sections.

The effective extension of the plasma display technique to providevariable intensity, or grey scale implies the existence of twoproperties. First a means must exist by which the intensity of a cellcan be varied. Second it must be possible to sustain different cells inan array at different intensities. The first property, by itself, wouldmake possible only a way of controlling the overall brightness of animage on the plasma display panel. The second property, however, makespossible the shading that id characteristic of images produced by halftone printing or displayed on television.

Several techniques for obtaining grey scale have already been describedin the above-mentioned copending application. One of these replaces eachcell by a group of cells from which the light is selectively filtered.With n cells in each group it is possible to obtain 2" brightnesslevels.A second technique allows control of the number of discharges that occurin a basic time interval-TN. field time for example. Unlike the cellgroup technique, the present invention does not increase the number ofrequired cells. It is, however, most applicable to'applications wherethe image is changed, or in renewed, after each basic interval.

In accordance with the present invention there is provided apparatus anda method wherein the intensity of each cell can be set independently andcan be' sustained indefinitely just as the single intensity is nowsustained. This approach depends on two properties of the plasma displaycell. First, the intensity of a discharge is related to the form of theexciting waveform at the time of firing and to the time during whichcharged particles are collected. For some cells this relation can be ex-0 pressed as a function of the slope of the exciting signal.

Second, with a properly shaped waveform the plasma display cell canexhibit a number of discrete stable states, each corresponding to adifferent slope of the exciting voltage, and therefore to a differentintensity. To understand this invention, we first review some principlesof the plasma display cell itself.

The cell, in the on state, fires once each half cycle, when the excitingvoltage and the wall voltage equal the firing voltage. In equilibriumthe charge transferred during each discharge is the same, and theexciting voltage at the times of successive discharges are equal inmagnitude, but opposite in polarity. This voltage is called therecurrent voltage. Any small change in wall charge causes acorresponding change in recurrent voltage, which because of the relationbetween voltage slope and total charge transferred to the walls, reducesthe original perturbation. This restoring effect is generally preciseand rapid. However, as will be hereinafter described in more detail,reduction of the perturbation can be made arbitrarily slow, it can alsobe made to grow exponentially, and at the limit of stability, it canpersist indefinitely. The deliberate introduction of a perturbation inthis case will thus change the stateand the intensity.

Although a quasi-stable state would be useful in a televisionlikedisplay in which a new picture was generated in each basic interval,only stable states are acceptable in the kind of information displaydescribed herein. As will be noted, however, regions of instability canalternate with regions of stability. Small perturbations from any stablestate will then damp out Small and any state once reached will persistindefinitely until it is changed by new information.

The invention will be better understood from the following detaileddescription thereof taken in conjunction with the accompanying drawingsin which:

FIG. 1 illustrates a driving or exciting voltage waveform illustratingthe alternation of stable and unstable sections or regions in thewaveform in accordance with the principles of the present invention.;

FIG. 2 is a schematic diagram illustrating one embodiment of apparatuswhich can be utilized to provide the ripped waveform of FIG. 1;

FIGS. 3, 4 and 5 are various waveforms useful in explaining theprinciples of the present invention;

FIG. 6 is a schematic diagram illustrating a plasma display panel drivenby suitable apparatus for providing the multilevel stable states inaccordance with one aspect of this invention;

FIGS. 7 and 8, illustrate a sustaining signal for the plasma displaypanel of FIG. 6, with the indicated change in wall voltage-in FIG. 7 thechange in wall voltage occurring at a point of high slope correspondingto a relatively bright or a high intensity cell, and in FIG. 8 thechange in wall voltage occurring at a point of relatively lower slope onthe sustaining signal waveform, corresponding to a relatively dimmer orless intense cell;

FIG. 9 illustrates apparatus for setting the wall charges in respectivecells corresponding to the intensity of the information to be displayed;and

FlGs. 10 and II are schematic diagrams illustrating respective outputsof the apparatus shown in FIG. 9 for providing corresponding waveformsreflecting the difference in intensity between a respective bright cellof high intensity (corresponding to FIG. 7) and a relatively dimmer cellof less intensity (corresponding to FIG. 8).

FIG. 1 shows the alternation of stable and unstable sections in theexciting voltage wave. This shape follows from what we have determined,wherein sections of the waveform of FIG. 1 with a positive secondderivative are unstable while sections with a negative sound derivative,if it is not too large, will be stable. Such sections are illustrated inFIG. 1. This network can be generated by driving an appropriate waveshaping network with a square wave. This technique together with arepresentative wave shaping circuit is shown in FIG. 2. As illustrated,the input to circuit 10 is a square wave of suitable amplitude andfrequency, and the output is a rippled wave of the form shown in FIG. 1with the alternating sections previously described.

The following is a detailed description of the stability considerationsconcerned in the concepts and teachings of this invention. Thefundamental relation in a discussion of the stability of firing times inthe plasma display cell is l', =V;V,, (1) where V and V, are themagnitudes of the wall voltage and the recurrent voltage (the excitingvoltage V at the beginning of the i discharge), and V, is the firingvoltage. The flow of charges to'the walls during the I discharge changesthe wall voltage by an amount 2V, to V and the next discharge in thesequence is ignited when the exciting voltage reaches the nextappropriate recurrent voltage.

ii-l (2) l r ll}- i-i-l FIG. 3 shows the relations among these voltagesfor an arbitrary exciting signal. We note, in particular, that since weare concerned with the magnitudes of these voltages, Vi, and w,

are always positive, and we, therefore, do not distinguish between thepositive and negative half cycles.

Two successive values of wall voltage are related by the expressionv..,,,= -v..,T- 3) and substituting from equation (1), we obtain therelation between two successive values of recurrent voltage V =2V; V,-2V (4) In terms of the difference 8, between V and the equilibriumvalue V and the difference 8 between V, and V this relation becomes Thequestion of stability rests on the relation between the form of theexciting signal, V, and the change in wall voltage, ZV This change, inturn, depends on the intensity of the discharge. In general theintensity increases with over voltage. Therefore, since the dischargerequires time to develop, the more intense discharge occurs when theslope of the exciting waveform at the time offiring is larger.

We assume now that the change in wall voltage can be expressed as afunction of slope. lf at equilibrium a perturbation from equilibriumdecreases to zero in time. For y beyond this range, the perturbation 8grows and the system is unstable.

To separate the influence of the charge-slope relation from that of thewaveform itself we replace the term df/dV by its equivalent dV dV dl dt(16) The first factor is a parameter of the cell that can be measured.The second term is the ratio of the time derivative of the slope to theslope itself.

It is instructive to consider the two cases at the limit of stability.When -y=0, a,- =a,. A perturbation 8 then alternates on successive halfcycles and persists indefinitely. This condition could arise if thecharge transferred were independent of the slope (df/dV=0), or if theslope did not change in the range that includes both the initialequilibrium voltages and the perturbed firing voltages. This second caseis illustrated in FIG. 4.

When y=-2, a a and a perturbation 8 once introduced, persistsindefinitely. The new firing times represent an equilibrium state justas the original firing times do. This case is illustrated in FIG. 5. Toobtain an acceptable form for this voltage wave we must find a solutionto the equation dV dt E If the charge transferred in a discharge werestrictly proportional to the slope (df/dV=k), then a solution would beV=C( l-C(Ie""/k) (18) If, as is more typical, afldV is not constant, butvaries smoothly with V, equation 18) is still an approximate solution toequation 17) over small ranges of V, provided that k is replaced by thelocal value of df ldVfor each range.

A third special case occurs when y=l. A perturbation for this case willbe cancelled in the next discharge in the sequence. For values of -y=l,therefore, the approach to equilibrium of a cell after change of statewill be rapid, a fact frequently observed in work with the plasmadisplay cell.

A practical implication of the above concepts and teachings is thatthrough control of the exciting voltage waveform, regions of stabilitycould be alternated with regions of instability. This would lead to amultiplicity of stable states and a corresponding number of intensities.

Referring to FIGS. 6 through 11 there is illustrated a specificembodiment of the present invention utilizing the teachings thereof toprovide a plasma display panel having multilevel stable states to obtaina display with variable intensity. As will be described in more detailin the following description, in general, the apparatus and methodsherein illustrated are provided for initially setting in an amount ofwall charge in each respective cell according to the intensity of theinformation to be displayed, and thereafter suitably discharging thecells having the respective wall charged by utilizing a sustainingsignal having a wave shape, in accordance with the principles previouslydiscussed so as to obtain the various intensity levels in the plasmadisplay with various respective stable states.

In particular in FIG. 6, there is illustrated the plasma display panel20 incorporating a gaseous medium, a first set of rows of electrodes 22on one side thereof, and a second set of columns of electrodes 24 on theother side thereof and disposed orthogonal to the electrodes 22 of thetype and as is described more completely in the previously mentionedcopending application and publications. Alternatively, since theprinciple of the plasma panel, as explained in the previously mentionedcopending application is the manipulation of the wall charges to impartinformation, the panel can be formed with one or both electrodesassociated with a cell electrically insulated from the gaseous medium.To each of the electrodes 22 there is connected a corresponding wallcharge setting means 26, which is its input receives the information tobe displayed in the form of an electrical signal having an amplitude orvoltage level corresponding to the respective intensity. For convenienceonly the first wall charge setting means 26 connected to the first rowelectrode 220 and the last wall charge setting means 28 connected to thelast row electrode 22e is shown in FIG. 6, however it is to beunderstood that the remaining electrodes in between electrode 22a and22s are connected to similar apparatus. Also, it is to be understoodthat the diagram illustrating the plasma display panel 22 in FIG. 6 isshown in a greatly enlarged view, since in most cases the illustratedcells are much closer together, typically being separated by about 25thousandths of an inch from cell center to cell center. The panel can ofcourse be of any suitable size, the illustration of five columns androws being merely for convenience here. While it may not be particularlyevident from FIG. 6, it is to be understood that the cells vary inintensity from the dimmest cells at the top left-hand corner of theplasma panel 20, namely cell 30, diagonally to the brightest cell 32 atthe bottom right-hand corner of the panel.

The sustainer 34 illustrated in block diagram form in FIG. 6 can beobtained from the apparatus shown in FIG. 2, wherein a square wave istransformed by the wave shaping network into the rippled wave of FIG. 1so as to obtain the multiple stable states. Thus, the sustaining signalshown in FIG. 7 and 8 is actually set in on all of the cells, that is,this signal represents the voltage difference between each of therespective column and row electrodes associated with the particularcells. It is to be understood that the principle underlying theobtaining of a variable intensity display is related to the fact thatthe level of intensity depends on the slope of the exciting signal atthe time of discharge, and that this in turn depends on the initial wallcharge. For example, referring to FIG. 7 there is represented thechanges in wall voltage of a bright cell such as the cell 32 in FIG. 6superimposed on a schematic representation of the rippled sustainingsignal voltage supplied by sustainer 34 to row electrode 22e andcolumnelectrode 24s. The vertical or ordinate axis of FIG. 7 has indicatedthereon an index mark above and below the zero abscissa axis to indicatethe voltage level of the initial wall charge, C which has been set intocell 32 by means of wall charge setting means 28 and the addresser 36,as will be described later in more detail. Also, the ordinate axiscontains index marks, V, indicating the required voltage differenceacross the respective cell electrodes in order to bring about adischarge of the gaseous medium within the cell. As the sustainingsignal is applied to cell 32, at a point indicated by reference number38, on the sustaining signal waveform, the potential difference betweenthe respective electrodes 22e,24e represented by the sum of thesustaining voltage and the voltage due to the initial wall charge equalsthe firing or discharge potential, V; so that the cell 32 discharges inwhat has been described as a pulsing discharge manner in the priormentioned copending application-that is, the discharge is quicklyextinguished by the formation of wall charges equal in amplitude to theinitial wall charges C,, but opposite in polarity thereto forming on thecell walls. In FIG. 7, such a condition is represented by the wallvoltage charging by an amount 2C or as indicated in the diagram to alevel C above the reference zero abscissa.

It is to be particularly noted that the cell has discharged at the pointin time indicated by the reference numeral 38 which is a point ofrelatively high slope on the sustaining signal waveform, which we havefound to produce a relatively brighter display than when the cell isdischarged at a point on the sustaining signal having a relatively lowerslope which, for the sustaining signal example shown shown in FIG. 7,would occur towards the top of the first symmetrical half of thesustaining figure waveform of FIG. 7. It must be understood that theabove description indicating what has been determined to be theattaining of a variable intensity as a function of the slope of thesustaining signal at the time of cell firing, relates to the overallaverage slope of the signal, and not to the very rapid and constantlychanging rippled condition of the slope, which of course, as previouslydescribed has been provided so as to insure the maintaining of stablestates in the plasma display. So as to completely understand thesituation, what might be described as the average slope of thesustaining signal for the first symmetric half of the waveform has beenindicated in dashed lines in FIGS. 7 and 8. Thus, it is clear that whenconsidering the dashed line representation of the average slopecondition of the sustaining signal at any particular time, the referencenumeral 38 on the sustaining signal waveform occurs at a point ofrelatively higher slope than that further along to the right and top ofthe first symmetric half of this waveform.

If the firing of the cell such as at reference point 38 occurs at anunstable region of the sustaining signal, the cell voltage adjustseither up or down the slope until the next stable region is encountered,and the cell wall voltage is thus locked in at that particular stablestate as described in the previous discussion above. In FIG. 8 there isillustrated a representation of a cell which is firing so as to displaya relatively dim or low level intensity as compared to the higherintensity representation of FIG. 7. For example, we may assume that thecell 30 associated with row electrode 22a and column electrode 240 hasbeen set with an initial wall charge corresponding to C by the wallcharge setting means 26 and addresser 36 in accordance with theinformation to be displayed at this particular point on the displaypanel 20. Thus, as shown in FIG. 8, as the sustaining signal applied tothe respective electrodes from sustainer 34 rises in voltage level, thewall voltage continues at the level corresponding to the initial wallcharge, C until a respective point in time indicated by the referencenumeral 40 on the sustaining signal where the sum of the wall voltagedue to the initial charge, C and the voltage due to the sustainingsignal equals in sum the firing voltage, V,. At this point, the pulsingdischarge situation occurs with a resulting buildup of wall charges ofopposite polarity in the cell 30 until a change in wall voltagecorresponding to 20,. occurs, or as indicated as in FIG. 8, until thewall voltage level corresponding to C has been reached above thereference zero abscissa. The discharge is then extinguished and the wallvoltage continues to the next firing point occurring on the nextsymmetric half cycle of the sustaining signal. It is to be noted herethat the firing point 40 occurs at a point of relatively lower slope onthe sustaining signal than the reference firing point 38 shown in FIG.7, thus the cell 30 fires with a lower intensity than the cell 32. Thewall voltage of cell 30 also adjust itself so as to stabilize at theclosest stable region on the sustaining signal.

Thus, a complete variation in intensity between a low and high level canbe provided by insuring that the cell will fire at a respective point onthe sustaining signal waveform, or in other words at a particular timecorresponding thereto. That is, if a bright high level intensity isdesired, the discharge should occur at a point of relatively higherslope, such as illustrated in FIG. 7, as compared to if a relativelydimmer display of lower intensity is desired which occurs at a point oflower slope on the sustaining signal. In other words, the desired valueor level of intensity for the displayed information can be provided byinsuring that the initial wall charge of the cell is such that thefiring of the cell will occur at the desired point along the sustainingsignal waveform. For example, notice that in the case of a bright cell,the initial wall charge should be large (corresponding to C, in FIG. 7),whereas for a relatively dim cell, the relative value of the wall chargeshould be less (C in FIG. 8).

Referring now to FIGS. 9, 10 and 12 there is illustrated one embodimentof the desired means for setting in of the wall charges" in therespective cells according to the level of intensity desired. In FIG. 9,there is illustrated what might be termed an integrator 42 whichreceives an input signal represented by the signal source 44 having anamplitude which represents the intensity of the signal desired to bedisplayed. This signal source 44 is coupled between the base and emitterof transistor 46 with suitable circuit components being provided so asto insure that the output of the transistor between the collector andemitter will be a waveform having a slope proportional to the amplitudelevel of the signal source 44. For example, if the input waveform V(t,)is supplied to the integrator 42, representing as an illustration thebright information which is to be displayed at cell 32, the resultingoutput of the integrator 42 is a triangular waveform S (see FIG. 10)whose slope is proportional to the amplitude or intensity of the inputsignal V(t Similarly, for an initial input signal of V(t representing alow level or dim signal which is to be displayed at cell 30 on panel 20,the resulting output of integrator 42 consists of a triangular waveformS (see FIG. 11) whose slope is proportional to the amplitude orintensity of the initial information which is to be displayed. Thus, theintegrator 42 is one illustration of a type of wall charge setting meanswhich can be used to set the wall charges of respective cells of theplasma panel 20.

The actual setting of the wall charges is obtained by synchronizing thevoltages placed on respective row electrodes 22 from the wall chargesetting means such as the integrator 42 and the voltages placed on thecolumn electrodes 24 by the'addresser 36. The addresser 36 provides asquare wave shaped selection or addressing signal having a periodcorresponding to the lowest intensity level to be displayed. Well-knownsync means 48 synchronize the placing of the respective voltages on therow and column electrodes. Thus, when the output signal S, is presentfrom wall charge setting means 28 on row electrode 22e, the addresser 36is synchronized so that at that time the proper selection signal iscoupled to the column electrode 24s to discharge cell 32 and set thewall voltage of cell 32 at a voltage level corresponding to a charge ofC representing the amplitude of the input signal V(t,). In a similarmanner, the wall voltage corresponding to a charge C is set in on cell30 by providing a suitable addressing or selection signal to the columnelectrode 24a, which together with the voltage corresponding to waveform5 on row electrode 22a fires the cell 30 and sets in the desired initialwall voltage.

It is to be understood, of course, that in accordance with theprinciples of the present invention, the multiple intensity andpermanent memory attained with this display apparatus can be provided byalternative embodiments. For instance, in connection with the means forsetting in the initial wall charges in the respective cells, it may bemore advantageous to employ other means than the integrator circuit 42described herein. An alternative technique for setting the wall charge,and therefore, the wall voltage to a desired value is to apply asuitable signal to the cell so that the voltage across the cell israised above the firing voltage, but while many charged particles remainin the volume, the voltage is reduced to the desired wall voltage level.The charged particles, attracted to the walls by the electric field,reduce the magnitude of the electric field, and, thereby, charge thewall voltage. Ideally, with a sufficiently large number of chargedparticles, the magnitude of the electric field will go to zero, and thewall voltage will become equal to the setting voltage.

Similarly, in connection with the rippled sustaining signal of the formshown in FIGS. 7 and 8, another alternative might be advantageous incertain situations. For example, note from the above discussion, and inparticular, equation 15, that the boundaries between a stable andunstable state can be defined in two ways. In one, the second derivativeof the cell voltage with respect to time actually changes sign. Thischange of sign is evident in the drawings of FIG. 1, FIG. 7, and FIG. 8.The value of the second derivative, however, need only change in a waythat allows the quantity to alternate between a value that is greaterthan 2 and a value that is less than -2. For a cell in which the chargetransferred were strictly proportional to the slope the waveforms wouldresemble those in FIGS. 1, 7, and 8, except that the perturbation fromthe smooth curve would be very small, and the second derivative wouldnot change sign. The circuit and technique illustrated in FIG. 2 wouldagain be appropriate, the components being chosen to provide a smalleramplitude for the ripple.

Whereas, in the above description of the present invention, there hasbeen illustrated the application of the multiple stable state techniquefor providing variable intensity in a display panel, it is to beunderstood that the principles of the invention can also be applied toother nondisplay situations. For example, the application of multiplestable regions can be utilized in transferring information identified inmultiple stable states.

The foregoing detailed description has been given for clearness ofunderstanding only, and no unnecessary limitations should be understoodtherefrom as modifications will be obvious to those skilled in the art.

What is claimed is:

1. In gaseous pulsing discharge display panel apparatus, including agaseous medium in said panel, and display points defined by associatedpaired electrodes, said display points including gaseous discharge cellshaving cell walls for forming wall charges thereon, wherein informationis transferred into and out of said panel by manipulating wall chargesassociated with the selective pulsing discharge of the gaseous medium atthe display points by coupling suitable exciting signals to theassociated paired electrodes, the improvement comprising means fordisplaying said information at variable intensity levels, including amultiple stable state sustaining signal generator coupled to saidelectrodes, said sustaining signal, having multiple stable regionsassociated therewith for discharging said gaseous medium in cooperationwith said wall charges so as to maintain said information related tosaid wall charges in said panel at said selected display points.

2. Display panel apparatus as claimed in claim 1, including wall chargesetting means for entering wall charges at respective display pointsproportional to the corresponding intensity level of said information,each of said stable regions related to a particular intensity level.

3. Display panel apparatus as claimed in claim 1, wherein saidsustaining signal generator comprises means for generating a rippledsustaining signal waveform having alternating portions with a negativesecond derivative and a positive second derivative, said negative secondderivative signal portions corresponding to said multiple stableregions.

4. Display panel apparatus as claimed in claim 3, wherein said means forgenerating a rippled sustaining signal waveform comprises a square wavegenerator and a wave-shaping network.

5. Display panel apparatus as claimed in claim 2, including addressingmeans for selecting and entering a value of wall charge at selecteddisplay points proportional to the level of intensity to be displayed atsaid display point, a bright display point having a higher wall chargeand being sustained at one stable region, and a relatively dim displaypoint having a lower wall charge and being sustained at another stableregion.

6. In gaseous pulsing discharge display panel apparatus, including agaseous medium in said panel, and display points defined by associatedpaired electrodes, said display points including gaseous discharge cellshaving cell walls for forming wall charges thereon, wherein informationis transferred into and out of said panel by manipulating wall chargesassociated with the selective pulsing discharge of the gaseous medium atthe display points by coupling suitable exciting signals to theassociated paired electrodes, the improvement comprising means formaintaining said information at various discrete levels, including amultiple stable state sustaining signal generator coupled to saidelectrodes, said sustaining signal having multiple stable regionsassociated therewith for discharging said gaseous medium in cooperationwith said wall charges so as to maintain said information related tosaid wall charges in said panel at said selected display points.

7. In the method of displaying video information in a gaseous pulsingdischarge display panel having a gaseous medium in said panel, anddisplay points defined by associated paired having multiple stableregions to said respective electrodes.

8. The method of displaying information as claimed in claim 7, fordisplaying video information of varying intensity levels, includingaddressing and simultaneously entering said wall charges at respectivedisplay points proportional to the cor-- responding intensity level ofsaid video information, said wall charges being maintained at saiddisplay points and being sustained in association with a respectivestable region.

@53 3 UNITED STATESPATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3D601 531 Dated August 24, 1971 Inventorzs) Donald L. Bitzer et al.

It is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

Column 2 line 39 delete "Small" and insert -e ponentia1ly- 1 Column 3,line- 3, "sound" should be "second".

Column 4,- line 33, "V=C(1- C(1-e /k) should be V=C(l-e /k) Column 6line 32 after "initial" insert wall Column 6, line 65 "12" should be-11.

Signed and sealed this 7th day or March 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK At testing Officer Commissioneroi Patents

1. In gaseous pulsing discharge display panel apparatus, including agaseous medium in said panel, and display points defined by associatedpaired electrodes, said display points including gaseous discharge cellshaving cell walls for forming wall charges thereon, wherein informationis transferred into and out of said panel by manipulating wall chargesassociated with the selective pulsing discharge of the gaseous medium atthe display points by coupling suitable exciting signals to theassociated paired electrodes, the improvement comprising means fordisplaying said information at variable intensity levels, including amultiple stable state sustaining signal generator coupled to saidelectrodes, said sustaining signal, having multiple stable regionsassociated therewith for discharging said gaseous medium in cooperationwith said wall charges so as to maintain said information related tosaid wall charges in said panel at said selected display points. 2.Display panel apparatus as claimed in claim 1, including wall chargesetting means for entering wall charges at respective display pointsproportional to the corresponding intensity level of said information,each of said stable regions related to a particular intensity level. 3.Display panel apparatus as claimed in claim 1, wherein said sustainingsignal generator comprises means for generating a rippled sustainingsignal waveform having alternating portions with a negative secondderivative and a positive second derivative, said negative secondderivative signal portions corresponding to said multiple stableregions.
 4. Display panel apparatus as claimed in claim 3, wherein saidmeans for generating a rippled sustaining signal waveform comprises asquare wave generator and a wave-shaping network.
 5. Display panelapparatus as claimed in claim 2, including addressing means forselecting and entering a value of wall charge at selected display pointsproportional to the level of intensity to be dIsplayed at said displaypoint, a bright display point having a higher wall charge and beingsustained at one stable region, and a relatively dim display pointhaving a lower wall charge and being sustained at another stable region.6. In gaseous pulsing discharge display panel apparatus, including agaseous medium in said panel, and display points defined by associatedpaired electrodes, said display points including gaseous discharge cellshaving cell walls for forming wall charges thereon, wherein informationis transferred into and out of said panel by manipulating wall chargesassociated with the selective pulsing discharge of the gaseous medium atthe display points by coupling suitable exciting signals to theassociated paired electrodes, the improvement comprising means formaintaining said information at various discrete levels, including amultiple stable state sustaining signal generator coupled to saidelectrodes, said sustaining signal having multiple stable regionsassociated therewith for discharging said gaseous medium in cooperationwith said wall charges so as to maintain said information related tosaid wall charges in said panel at said selected display points.
 7. Inthe method of displaying video information in a gaseous pulsingdischarge display panel having a gaseous medium in said panel, anddisplay points defined by associated paired electrodes, said displaypoints including gaseous discharge cells having cell walls for formingwall charges thereon, wherein said information is entered into the panelat selected display points by coupling addressing signals sufficient todischarge the gaseous medium to respective electrodes and formassociated wall charges, said wall charges entered in said panel relatedto said information, the improved step of sustaining said information insaid panel at various information levels by applying a multiple stablestate sustaining signal having multiple stable regions to saidrespective electrodes.
 8. The method of displaying information asclaimed in claim 7, for displaying video information of varyingintensity levels, including addressing and simultaneously entering saidwall charges at respective display points proportional to thecorresponding intensity level of said video information, said wallcharges being maintained at said display points and being sustained inassociation with a respective stable region.